1. Field of the Invention
The present invention relates generally to prevention of interchannel crosstalk in wavelength division multiplexing (WDM), and more particularly to an optical sender, terminal device, and optical communication system each having the optical sender suitable for application to WDM.
2. Description of the Related Art
Known as a terminal device for WDM using a plurality of wavelength channels is a terminal device including a plurality of optical senders for outputting optical signals having different wavelengths and an optical multiplexer for wavelength division multiplexing the optical signals output from the optical senders to output WDM signal light. The WDM signal light is transmitted through an optical fiber transmission line to another terminal device. By applying such WDM using a plurality of wavelength channels, a large-capacity optical fiber communication system can be provided.
In an optical fiber communication system adopting WDM, the number of wavelength channels includable in a given wavelength band can be increased by reducing the spacing between adjacent wavelength channels. To achieve a larger capacity of the WDM system, a reduction in interchannel crosstalk is therefore required.
Each optical sender includes a light source for generating an optical signal per channel. In general, the light source is a laser diode for direct modulation or indirect modulation. The oscillation wavelength of the laser diode is determined by its temperature and drive current.
Due to the characteristics of the laser diode as mentioned above, the wavelength of an optical signal to be output from each optical sender tends to be unstable. For example, in starting up or shutting down the system or in the case of a trouble in relation to temperature control or the like performed in each optical sender, there is a possibility that the wavelength of the optical signal may be deviated from a target wavelength assigned to the corresponding channel, causing the occurrence of interchannel crosstalk. The interchannel crosstalk creates a problem that transmitted data cannot be accurately demodulated in a terminal device for receiving WDM signal light.
To prevent the interchannel crosstalk, additional provision of an optical bandpass filter in each optical sender may be proposed. The optical bandpass filter has a passband including the wavelength of the corresponding channel and excluding the wavelengths of the other channels. Accordingly, even when the wavelength of the optical signal is deviated from the target wavelength assigned to the corresponding channel, the crosstalk to the other channels can be prevented.
However, the additional provision of the optical bandpass filter in each optical sender makes complicated the configuration of a terminal device for transmission and invites a cost increase. Furthermore, the power of the optical signal obtained is reduced by insertion loss due to the optical bandpass filter.